"What they're talking about is giving you the right to make less money," Obama told union members at an engine plant near Detroit.

In discussing his economic plan, Obama told the auto workers: "What we shouldn't be doing is try to take away your rights to bargain for better wages or working conditions."

Right-to-work law pending in Michigan's Republican-run legislature "don't have to do with economics," Obama said. "They have everything to do with politics."

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President Obama will likely discuss his fiscal cliff plan to raise taxes on the wealthy when he visits the Detroit area on Monday.

He is also expected to hail his host -- Daimler, owner of the Detroit Diesel Corp. -- for a new investment plan to expand production and create jobs in the United States.

But how much will Obama say about Michigan's ongoing, massive labor battle?

Obama arrives in the nation's automotive capital just as the lame-duck, Republican-run state Legislature is poised to pass right-to-work legislation. The prospect has triggered protests from Michigan's unions.

Gov. Rick Snyder, a Republican, has indicated he would sign the bill that would make the payment of union dues voluntary.

The White House has already criticized the legislation in a written statement. Said spokesman Matt Lehrich:

"President Obama has long opposed so-called 'right to work' laws and he continues to oppose them now. The president believes our economy is stronger when workers get good wages and good benefits, and he opposes attempts to roll back their rights. Michigan -- and its workers' role in the revival of the U.S. automobile industry -- is a prime example of how unions have helped build a strong middle class and a strong American economy."

Unions gave Obama major support in both of his election victories. This was especially true in Michigan, where nearly one-fifth of the state's adults belong to unions (though that is down from 40% during the 1960s).

It's hard to imagine Obama won't say something about what is shaping up as the biggest union battle of the next election cycle.